1. Field of the Invention:
The present invention relates to a method for producing a light emitting diode (hereinafter referred to as an `LED`), and more particularly to a method for producing an LED to be used for display and transmission purposes.
2. Description of the Related Art:
There has been a demand for a bright LED to be used for many applications including a display apparatus. One approach to make a bright LED is to improve the external emission efficiency of the light emitted inside an LED.
An LED is made of semiconductor materials having high refractive indices, e.g. approximately to the order of 3.5, which means total reflection can easily occur. Therefore, in the case where light is emitted from one plane face of the LED, only light which enters into the plane face at an angle less than the critical angle can propagate to the outside. Therefore, because of the total reflection, the external emission efficiency of the LED is usually not so high.
A common method for improving the external emission efficiency of an LED is to use a material capable of transmitting the generated light for a semiconductor substrate and to form a reflector on the back-side of the semiconductor substrate. In such an LED having a substrate transparent to the generated light, light reflected from the lower face of the semiconductor substrate can be emitted from the upper face, side faces, etc., whereas only the light emitted from the upper face can be utilized in cases where the semiconductor substrate is made of a material which does not transmit the generated light. LEDs having a substrate that is transparent to the generated light are realized as infrared LEDs in which a semiconductor material of an InGaAsP-type is used, infrared and red LEDs in which a semiconductor material of an AlGaAs-type is used, yellow LEDs in which a semiconductor material of a GaAsP-type is used, and green LEDs in which a semiconductor material of a GaP-type is used.
On the other hand, there have been developed red, yellow, and green LEDs in which a semiconductor material of an AlGaInP-type is used, and green and blue LEDs in which a semiconductor material of a ZnCdSSe-type is used. However, the above-mentioned technique of forming the substrate with a material transparent generated light is not applicable to these LEDs, because the lattice-matching conditions are not satisfactory in such applications. Moreover, the internal emission efficiency of the above-mentioned LEDs, in the case where a substrate that is opaque to generated light is used, tends to decrease because the substrates opaque to generated light are not suitable for these LEDs.
The internal emission efficiency of these LEDs can be improved by constituting their semiconductor substrates with a material opaque to generated light and growing the above-mentioned semiconductor materials on the substrates. However again, the external emission efficiency of such LEDs are inevitably lowered since the substrates that are opaque to generated light are used, as is described above.
Japanese Laid-Open Patent Publication No. 3-274770 discloses an LED having the configuration shown in FIG. 13, in which the above-mentioned problems are solved. The LED is fabricated in the following manner: First, are an n-AlInP first cladding layer 901, an undoped AlGaInP light emitting layer 902, a p-AlInP second cladding layer 903, a p-AlGaInP intermediate band gap layer 904, and a p-AlGaAs retaining layer 905 are grown on a first n-GaAs substrate 900 in this order, by the use of an MOCVD (Metal Organic Vapor Phase Epitaxy) method. Next, the first n-GaAs substrate 900 is removed, and electrodes are formed. Thereafter, the LED structure is mounted on a stem, the p-AlGaAs retaining layer 905 being disposed at the bottom.
However, the above-mentioned fabrication method has the following problems: Firstly, an LED provided by this method requires the p-AlGaAs retaining layer to be formed very thick so that the overall LED can maintain physical strength after the removal of the first n-GaAs substrate 900. This requires too much time for the formation of the p-AlGaAs retaining layer 905 in cases where the above-mentioned MOCVD method is used. Secondly, in the case of an LED in which an Al.sub.x Ga.sub.1-x As (0.ltoreq.X.ltoreq.1) retaining layer is used, it is necessary to set the content rate X at a value near 1 in order to ensure that the retaining layer is transparent even in cases where the generated light is of a short wavelength such as that in the green band. However, this leads to the problem in that the retaining layer is susceptible to oxidation in air, thus becoming chemically unstable.